Using a 14-day trial of GP Ultimate when I already have Gold membership

My son recently replaced his controller after the mic jack of BOTH controllers that came with our game system broke within 4 months of starting to use mic/headphones (THAT is a design flaw issue I should take up with you, Microsoft, another day. He's a young
player.. And those controllers went down during the peek of COVID stay-at-home behavior, so he ended up having to spend about 2 years of birthday money on a basic controller + shipping...'cause there were none available anywhere locally.)

In the box with his new controller was a code for a 14 day trial of Game Pass Ultimate. And he wants to use it. But when I helped him try to redeem the code on his account, a warning displayed about converting all of the remaining membership to the Ultimate
trial and there being no way to reverse that conversion. It advises to go to www.xbox.com/gamepass for more details, but of course questions about this particular issue are not covered.

I am afraid he will lose more than 9 months of our pre-paid Gold membership and gain only 14 days of GP Ultimate. Is this the case?

Thanks in advance for any help with this!

:)